The present invention relates to a centrifugal extractor capable of rapidly performing liquid-liquid extraction by utilizing centrifugal force and, more particularly, to a high-speed centrifugal extractor equipped with improved weirs for facilitating selective extraction of heavy liquid and light liquid.
The high-speed centrifugal extractor is particularly suitable for separating uranium and plutonium contained in spent nuclear fuel from nuclear fission products when reprocessing the spent nuclear fuel by a solvent extraction method (such as a Purex process), but is not particularly limited to this field and can be applied widely to fields requiring liquid-liquid extraction of a heavy liquid and a light liquid.
A reprocessing method of spent nuclear fuel using the Purex process involves the steps of bringing a nitric acid solution (heavy liquid) containing uranium, plutonium and nuclear fission products into counter current flow contact with a tributyl phosphate (hereinafter called "TBP") diluted with hydrocarbon solution (light liquid) as an extraction solvent of uranium and plutonium in order to extract uranium and plutonium into the TBP from the nitric acid solution, then bringing the TBP once again into counter current flow contact with a new nitric acid solution in order to remove and wash the nuclear fission products that have been extracted in slight amounts into the TBP, and further bringing this washed TBP into counter current flow contact with a dilute nitric acid solution to cause a reverse extraction of the uranium and plutonium in the TBP into the dilute nitric acid solution.
Extractors such as a mixer-settler, a pulse column, and the like, have been used generally for the extraction, washing and reverse extraction processes described above. In the mixer-settler, however, a sufficient residence time must be secured because natural gravitational force acting on the liquids having different specific gravities is utilized in order to separate both liquids inside the extractor, and for this reason TBP as the extraction agent is likely to be damaged by radioactive rays. In the pulse column, on the other hand, it is known in the art that the dispersion state is deteriorated due to the wettability of the perforated plate fitted inside the column. These are the technical problems to be solved in order to obtain a high decontamination factor and a stable operating condition. In order to increase the processing capacity, a floor area must be increased in the mixer-settler while the diameter and height of the column must be increased in the pulse column. This means that the overall size of the extractors must be increased.
A high-speed centrifugal extractor has recently been developed as an extractor that solves the problems of the conventional extractor described above. The high-speed centrifugal extractor forcibly separates a mixture of the heavy liquid and light liquid into the heavy liquid and the light liquid by centrifugal force, and its typical construction is shown in FIG. 5 of the accompanying drawings. The high-speed centrifugal extractor fundamentally comprises a casing 51 and a cylindrical rotor 52 that is rotated at a high speed by a rotary shaft 53 inside the casing. The heavy liquid (such as a nitric acid solution) and a light liquid (such as TBP as an extraction agent) are supplied into a mixing chamber 56 at a lower part of the casing 51 from respective supply pipes 54 and 55. After they are sufficiently mixed inside the mixing chamber 56 by an impeller 57 disposed at the lower end of and rotating with the rotary shaft, the mixture is introduced into the rotor 52 from a center opening 59 of a rotor lower end plate 58. After the mixture is further stirred between the rotor end plate 58 and a baffle plate 60, the heavy liquid having a greater specific gravity is separated outwardly while the light liquid having a smaller specific gravity is separated inwardly of the heavy liquid due to the centrifugal force at the inner peripheral surface 52a of the rotor, and the liquids rise upwards along the rotor inner peripheral surface. Weirs 61, 62 for selectively extracting the liquids are disposed at the upper inside part of the rotor 52 to separately withdraw the heavy liquid and the light liquid through heavy and light liquid outlets 63 and 64, respectively. The heavy liquid selection weir 61 has a heavy liquid draw port 61a that opens outwardly of the interface K between the outer heavy liquid phase and the inner light liquid phase, that is, on the side of the heavy liquid phase. The heavy liquid passing through this port 61a overflows over a plurality of weir plates 61b, 61c, 61d, is then fed to the heavy liquid outlet 63 and is discharged through a heavy liquid discharge port 66 from a heavy liquid collection chamber 65 (as shown by the solid line arrow in the drawing). On the other hand, the light liquid selection weir 62 has a light liquid draw port 62a that opens inwardly of the interface K between the heavy liquid phase and the light liquid phase, that is, on the side of the light liquid phase, and the light liquid overflowing into this draw port 62a is introduced into a light liquid outlet 64, and is thence discharged through a light liquid discharge port 68 from a light liquid collection chamber 67 (as shown by the dotted line arrow in the drawing).
Since the high-speed centrifugal extractor forcibly separates the heavy liquid and the light liquid by centrifugal force as described above, it provides the following advantages.
(1) Since mixing is performed at a high-speed, the extraction efficiency is high.
(2) Since the contact time is extremely short, damage of the extraction agent by radioactive rays is minimal.
(3) Since the quantity of the liquid residing inside the extractor is small, the hold up quantity of nuclear substances and radioactive substances is small.
(4) The size of the extractor required in order to provide the same processing capacity as that of the conventional pulse column or mixer settler is extremely small.
(5) The time required before operation the operation equilibrium state is reached is short so that the time between the start and stop of processing is extremely short and also the quantity of resulting waste liquor is extremely small.
In the centrifugal extractor having the structure described above, the weirs 61, 62 for selecting the heavy liquid and the light liquid are fixed between the rotor inner peripheral plane 52a and the rotary shaft 53. On the other hand, the position of the interface K between the heavy liquid phase and the light liquid phase separated by the centrifugal force inside the rotor changes in accordance with the operating conditions, and for this reason, a charge in the position of the interface K must be suitably accomodated for with a change in the operating conditions in order to always obtain a high separation function of separating heavy liquid and light liquid by the weirs 61, 62 having a predetermined height.
In the conventional centrifugal extractor described above, compressed air that is pressure-fed from outside the extractor through a passage 70 inside the rotary shaft 53 is used as means for controlling the interface K. In other words, the compressed air is introduced, whenever necessary, into a sealed chamber 80 through the passage 70 to increase the air pressure inside the sealed chamber 80 and to control the interface K inside the rotor 52.
However, a complicated seal structure is necessary in order to reliably feed the compressed air without leakage through the rotary shaft 53, but the seal structure is likely to allow air leakage during a long period of use so that reliable control of the interface becomes impossible.
Furthermore, the structure of the selection weirs 61, 62 per se is complicated, too, because a plurality of weir plates 61b-61d are fixed to the rotary shaft 53 and the rotor inner peripheral plane 52 in such a manner as to project therefrom. Since they must be accurately balanced for allowing high speed revolution, a high level of skill is necessary for producing them.